If you would like to unlock this achievement go into a skirmish battle and just build an army of 100 units, any units will do.

Store them all in one place and then when your unit count is 100 or more select them all at the same time and order them to move or attack.
Make sure to select your avatar also as it counts as a unit. Then the achievement should pop.

To make it easier set the difficulty to easy and have a couple AI avatars auto attack the enemy, they don't count to the unit total and keep the enemy distracted.

You also don't even have to finish the fight after that, I quit to main menu to check if it worked and I had no problem with it.